Personal computers handle mostly parallel data with byte length and word length. To transmit parallel data through a cable, the cable is needed to be a multiple wire cable including eight wires for byte length data or sixteen wires for word length data. However, it is not cost effective to use the multiple wire cable especially when a transmission length becomes relatively long. Since a serial data communication method contributes reduction of the number of wires, variety of serial communication methods have been proposed. When byte data is transmitted, the byte data is broken down to bit data and the bit data is then transmitted.
FIGS. 1 to 4 show general methods of serial data communications. FIG. 1 illustrates one example method in which two signals are used. One of the two signals is a data signal SdA to represent data simply as levels of the signal. The other signal is a synchronizing signal SaA which represents boundaries of the data and extracts the data from the data signal SdA by a rising edge of the synchronizing signal SaA. In this method, at least two signal wires are needed.
FIG. 2 illustrates another method which is called a pulse width modulation method. Signal SdB is modulated with regard to a pulse width and changes a time period of high level. A pulse width for data “1” differs from a pulse width for data “0”. However, a relatively large circuit is necessary to perform a complicated procedure of the pulse width modulation.
FIG. 3 illustrates another method which is called a pulse position modulation method. In this method, each pulse has the same width as others but positions of pulses of a data signal SdC are changed depending on the data. Sampling of the data signal SdC is performed using a synchronization signal SaC which represents a base time. Also, a relatively large circuit is necessary to perform a complicated procedure of the pulse position modulation.
FIG. 4 illustrates another method which is commonly used in infrared remote-control systems. This method is a combination of the width modulation method and the pulse position modulation method. Time intervals from data to next data in a data signal SdD are not constant. Thus, the data signal SdD is a kind of an asynchronous signal which requires no clock signal. However, this method requires a relatively large circuit to perform the complicated function.
Among the serial communication methods, a half-duplex channel communication method is frequently used because of its simplicity. FIG. 5 illustrates a block diagram of a conventional serial communication apparatus 200 using the half-duplex channel communication. The serial communication apparatus 200 includes a master transmitter-receiver circuit 201, a slave transmitter-receiver circuit 205 and a transmission path 209. The master transmitter-receiver circuit 201 includes a master transmitter circuit 202, a master receiver circuit 203 and a master control switch 204. The slave transmitter-receiver circuit 205 includes a slave transmitter circuit 206, a slave receiver circuit 207 and a slave control switch 208.
The slave transmitter circuit 206 is similarly configured to the master transmitter circuit 202. The slave receiver circuit 207 is also similarly configured to the master receiver circuit 203. Each of the master and slave control switches, 204 and 208, controls possession of transmission right which determines a transmission direction through the transmission path 209 between the master transmitter-receiver circuit 201 and the slave transmitter-receiver circuit 205.
When the master transmitter-receiver circuit 201 holds the transmission right, data is transmitted from the master transmitter circuit 202 to the slave receiver circuit 207. On the other hand, when the transmission right is held by the slave transmitter-receiver circuit 205, data is transmitted from the slave transmitter circuit 206 to the master receiver circuit 203.
In such conventional serial communication apparatus, relatively large slave circuits are required to be able to operate the same functionality as a circuit in the master side. Moreover, complicated switching circuits are needed to switch the direction of the transmission. Thus, there are increasing demands for an improved technique for transmitting serial data with a smaller circuit and less wires.